This invention relates generally to prismatic pouch cell batteries, and more particularly to coated aluminum battery cells tabs.
In the battery industry, battery cell tabs are joined together to create a low resistance joint. Reversible joining methods are desirable because they allow easy assembly and disassembly of battery cell modules. Reversible compression joining is a preferred method of assembly for cell tabs, provided the joint has low electrical resistance and long-term reliability.
Aluminum oxide naturally occurs on the surface of bare aluminum exposed to oxygen. The oxide layer protects the underlying metal from further oxidization (similar to rust). This naturally occurring oxide layer is not uniform in thickness or structure. The oxide layer reduces the surface conductivity of the aluminum.
Aluminum is commonly treated by a process of anodization, which grows a uniform oxide layer on the surface of the metal. Cell manufacturers use the anodized coating to protect aluminum cell tabs from chemical attack and to improve other attributes of the cell. The anodized coating is uniform and covers the entire cell tab, the portion outside the cell as well as the portion inside the cell.
The anodized coating is also an electrical insulator which increases resistance when joining cell tabs. The inherent resistance of the anodized coating limits the possible joining methods that can be used. For example, an anodized aluminum compression joint has about 250 micro-ohms of resistance, which is too high for most applications. An acceptable joint resistance is typically about 10 to about 50 micro-ohms. High resistance cell tab joints generate excess heat and reduce overall battery performance.
In order to obtain the low resistance needed for an aluminum cell tab joint, the anodized layer must be removed. Ultrasonic, laser, and resistance welding use high energy processes to remove or break up the oxide layer. The difficulty with laser and resistance welding processes is that when fusion joining copper (e.g., a copper buss) to aluminum cell tabs, the resulting joint is often brittle because of the alloying effect of mixing metals in a molten state. In order to avoid these problems, ultrasonic welding has been used. However, if too much energy is used when joining the cell tabs with ultrasonic welding, the excess energy is transmitted into the cell and can damage its delicate electrical connection to the cell tab. In addition, none of these joining methods are reversible, and they do not allow for replacement of a single cell. Consequently, if one cell fails, the entire module must be replaced, which significantly increases manufacturing and operating costs.
Soldering is also a common joining method. In this case, an acid flux is used to remove the oxide chemically prior to the soldering process. This process is reversible; however, the acid flux used to clean and remove the oxides for soldering, as well as the high heat, can cause issues with the cell seal around the cell tab, potentially causing electrolyte leakage. This also adds additional processes cost for manufacturing.
Therefore, there is a need for an improved cell tab and method of joining cell tabs.